The solution polycondensation of a mixture of various parts of isophthalic acid (IPA) and terephthalic acid (TPA) with 4,4'-dihydroxydiphenylsulfone (BPS) with tosyl chloride/dimethylformamide/pyridine as a condensing agent was studied. To elucidate how the reaction should be done to obtain copolymers of high molecular weights, we examined the two-stage copolycondensation with BPS and 2,2-bis(4-hydroxyphenyl)propanes (BPAs) by changing the content of IPA/TPA and the amount of BPS or BPA used in the initial reaction. Controlling the reaction at an earlier stage could facilitate the copolycondensation. The polyesters of IPA/TPA and BPS of moderate inherent viscosity values up to 1.0 were obtained by the two-stage reaction with optimal amounts of BPS first and then additional BPS. Satisfactory results were also obtained by the dropwise addition of BPS over 10-20 min in the preparation of the IPA/TPA-BPS polymers containing less than 50 mol % TPA.